In addition to the points noted in other answers, there is a significant technical difference: bearings. Traditionally, Dura-Ace equipment with bearings has had much better quality bearings/races/cups/seals/etc. That appears to be true here as well. Look at the parts breakdown documents from shimano and you'll see a significant difference in the ...

According to this site, the differences can be summed up in the following
As for the carbon pedal body, the 6800 design differs from 9000 through its use of short carbon fibers instead of Dura-Ace's long carbon fibers. Essentially, this is where the extra weight comes from. Additionally, the carbon has been molded with a machined aluminum sleeve that ...